1. Field of the Invention
The present invention relates to an internal combustion engine provided with a decompression mechanism incorporated into a camshaft included in a valve train and disposed in a valve chamber. The internal combustion is intended for use as, for example, an outboard engine.
2. Description of the Related Art
A multicylinder internal combustion engine intended for use as an outboard engine disclosed in, for example, JP2000-227064A (FIGS. 4 and 5) is a two-cylinder internal engine provided with a decompression mechanism to facilitate an engine starting operation. This two-cylinder internal combustion engine is provided with a camshaft disposed in a cam chamber defined by a cylinder head and a cylinder head cover, intake and exhaust cams formed on the camshaft to operate intake valves and exhaust valves, rocker arms driven for a rocking motion by the intake and exhaust cams, a decompression lever mounted on the camshaft so as to be turnable in a vertical plane under the exhaust cams, and a fuel pump. In this internal combustion engine, the exhaust cam in contact with the contact part of the rocker arm, and the end of the stem of an exhaust valve in contact with the pushing part of the rocker arm are at the same position with respect to a direction parallel to the axis of the camshaft, and hence the rocker arm extends perpendicularly to the axis of the camshaft. The decompression lever has an upper part provided with a decompression cam, and a lower part provided on its opposite sides with weights. The weights are moved radially outward by centrifugal force as the engine speed increases. 
A multicylinder internal combustion engine, intended for use as an outboard engine, disclosed in JP3-3904A is a three-cylinder internal combustion engine provided with a camshaft supported in four camshaft bearings on a cylinder head forming a valve chamber, and cams formed on the camshaft to rock rocker arms (hereinafter referred to as “valve cams”). In this multicylinder internal combustion engine, the valve cam in contact with the contact part of the rocker arm, and the end of the stem of an intake or exhaust valve in contact with the pushing part of the rocker arm are at the same position with respect to a direction parallel to the axis of the camshaft, and hence the rocker arm extends perpendicularly to the axis of the camshaft. This known three-cylinder internal combustion engine has the camshaft, the valve cams, and a fuel pump driven by a driven rod driven for axial movement by an eccentric cam formed on the camshaft. The fuel pump is attached to a side surface of the cylinder head.
It is difficult to secure an axial space for mounting a decompression lever mentioned in JP3-3904A on the camshaft extending across the three cylinders of the internal combustion engine disclosed in JP3-3904A. Consequently, the length of the camshaft needs to be increased and thereby the axial length of the valve chamber needs to be increased accordingly.
As mentioned in JP3-3904A, the driven rod serving as a pump-operating member in contact with the eccentric cam, i.e., a pump drive cam, is used to transmit the driving force of the eccentric cam to the fuel pump. When the decompression lever mentioned in JP2000-227064A is disposed contiguously with the pump drive cam, the decompression lever and the pump drive cam must be arranged so that the swinging decompression lever may not interfere with the pump-operating member. Consequently, the length of the camshaft needs to be increased and thereby the axial length of the valve chamber needs to be increased accordingly.
The present invention has been made in view of such circumstances and it is therefore an object of the present invention to secure a space for placing a decompression mechanism in an internal combustion engine without increasing the length of a camshaft extending across three or more cylinders and the axial size of a valve chamber, and to form a multicylinder internal combustion engine in compact construction. Another object of the present invention is to ensure stable operation of a valve train included in a multicylinder internal combustion engine during the operation of the multicylinder internal combustion engine at high engine speeds.
A further object of the present invention is to avoid interference between a pump drive cam and a decompression mechanism, suppressing increase in the length of a camshaft extended in a valve chamber and provided with a pump drive cam and decompression mechanisms and in the axial size of the valve chamber, and to form a multicylinder internal combustion engine in compact construction.